Yes, this is quite easy to do. First, create your symbol using a bitmap or icon editor. MapPoint accepts uncompressed Windows bitmap (.bmp), icon (.ico), and cursor (.cur) as custom symbols. The maximum size limit is 128x128 pixels and the color depth can be 2, 16, 256, or 16777216 (24-bit). Use the import custom symbol button to import your image (see below):

<center>[img]/images/mappoint_custom_symbol.jpg[/img]</center>